Aridity and vegetation composition are important determinants of leaf-wax δD values in southeastern Mexico and Central America

Leaf-wax hydrogen isotope composition (δDwax) is increasingly applied as a proxy for hydroclimate variability in tropical paleoclimate archives, but the factors controlling δDwax in the tropics remain poorly understood. We measured δDwax and the stable carbon isotope composition of leaf-waxes (δ13Cwax), including both n-alkanes and n-alkanoic acids, from modern lake sediments and soils across a marked aridity gradient in southeastern Mexico and northern Central America to investigate the importance of aridity and vegetation composition on δDwax. In this region the estimated hydrogen isotope composition of meteoric water (δDw) varies by only 25‰, and variability in δDw does not explain the relatively large variance in δDwax (60‰). Instead, the aridity index, defined as the ratio of mean annual precipitation to mean annual potential evapotranspiration (MAP/PET), explains much of the variability in the hydrogen isotope fractionation between leaf-waxes and meteoric water (εwax/w). Aridity effects are more evident in lake sediments than in soils, possibly because integration of leaf-waxes across a broad catchment masks small-scale variability in εwax/w that is a consequence of differences in vegetation and microclimates. In angiosperm-dominated environments, plant ecology, inferred from δ13Cwax, provides a secondary control on εwax/w for n-alkanoic acids (εn-acid/w). Low δ13Cn-acid values are associated with high εn-acid/w values, most likely reflecting differences in biosynthetic hydrogen isotope fractionation between C4 grasses and C3 trees and shrubs. A similar relationship between δ13Cn-alkane and εn-alkane/w is not observed. These results indicate that changes in either aridity or vegetation can cause large variability in δDwax that is independent of the isotopic composition of precipitation, and these effects should be accounted for in paleoclimate studies.